The lawn care or landscape maintenance industry uses spreaders to apply products, such as fertilizer and other granulated material, to maintain and promote a healthy lawn, for example. Often the landscaper or lawn care worker must apply these products to a lawn in a climate that changes over the course of a calendar year. For example, in some geographic areas the spring season may have a climate with relatively low humidity and low temperatures compared to its summer season that may have a climate with relatively high humidity and high temperatures. The level of humidity typically affects the fertilizer or granulated material in the spreader such that the fertilizer or granulated material will clot or clump in the spreader, thereby making it difficult for the material to pass smoothly through the spreader to be distributed onto a lawn.
Often spreaders have a shut-off plate or control gate that controls the rate that a fertilizer is applied to a lawn. The shut-off plate may be attached to, and controlled by, a solenoid which may work in combination with a spring. Similar problems exist with solenoid-spring operators in other applications. Solenoid-spring operators move an operator between two positions. The spring acts on the operator tending to move the operator in a first direction (usually a “closed” position). The solenoid, when energized tends to move the operator in a second direction generally opposite the first direction (usually an “open” position). To reach the open position, the solenoid must overcome the force applied to the spring. When the operator is in the open position, the spring is stretched. When the solenoid is deenergized removing the force overcoming the spring, the spring moves the operator to the first position. Over time as the spring ages and wears out, it applies a lesser force against the force of the solenoid. As a result, the operator moves further in the “open” position and the spring does not return the operator to the “closed” position.
It is also difficult to balance the rate of closure of the shut-off plate spring with the opening power of the solenoid in both humid climates (where the fertilizer becomes sticky) and dry climates (where the fertilizer remains granular). In these situations, too much solenoid power, or too weak a spring, may prevent the spring from efficiently closing the shut-off plate in dry weather, while too little solenoid power, or too strong a spring, may not allow the shut-off plate to open properly in humid weather. Either situation may result in an improper amount of material being spread or applied. For that reason, spreaders and other devices having a solenoid and spring combination have not proven to be completely satisfactory, and improved means of operating a shut-off plate on spreaders is needed. It is desirable that a spreader be capable of delivering an accurate amount of fertilizer or other material with repeatable opening and closing of the shut-off plate in any environment.